The present invention relates to an organic electroluminescence display apparatus.
An organic electroluminescence (hereinafter referred to as “organic EL”) display apparatus is formed by arranging a plurality of elements, each being essentially of 3-layer structure with an organic light-emitting layer placed between an anode and cathode electrodes, the former for injecting holes and the latter for injecting electrons into the intermediate layer.
The organic light-emitting layer is of (1) 2-layer structure with an emitting layer and hole transport layer in this order from the cathode electrode side, (2) 2-layer structure with an electron transport layer and emitting layer in this order from the cathode electrode side, or (3) 3-layer structure with an electron transport layer, emitting layer and hole transport layer in this order from the cathode electrode side. When a voltage of several volts is applied to the element between the electrodes, holes are injected from the anode electrode and electrons from the cathode electrode, and they are combined with each other in the emitting layer to emit light.
Display apparatuses with an organic EL element fall into two types, one is driven by a simple matrix and the other by an active matrix. An organic EL display apparatus driven by a simple matrix has organic light-emitting layers positioned at the intersections of anode lines with cathode lines, and each pixel is on only for a selected time in one frame period. Selected time is a time span of one frame period divided by number of the anode lines. An apparatus of this type has an advantage of simple structure.
However, selected time decreases as number of pixels increases. In order to secure a given average brightness in one frame period, therefore, it is necessary to increase instantaneous brightness in a selected time span by increasing driving voltage. This causes problems resulting from shortened service life. An organic light-emitting element, which is driven by current, suffers voltage drop caused by wiring resistance, in particular in the case of large screen. As a result, voltage cannot be applied uniformly to pixels, deteriorating brightness uniformity in the apparatus. Therefore, there are limitations of precision and screen size for an organic light-emitting display apparatus.
In an organic light-emitting display apparatus driven by an active matrix, on the other hand, 2 to 4 driving elements, each composed of a switching element of thin-film transistor and capacitor, are connected to an organic light-emitting element which constitutes a sub-pixel. As a result, all pixels can be on in a frame period to extend service life of the organic light-emitting element, because necessity for increasing brightness is removed. Therefore, an active matrix type organic light-emitting display apparatus is more advantageous over a simple matrix type, when high precision and large screen are required.
A conventional organic light-emitting display apparatus has a bottom emission structure with light emitted from the back side of the substrate. This structure limits aperture ratio of an active matrix type organic light-emitting display apparatus with a driving section between the substrate and organic light-emitting element.
One attempt to solve these problems is adoption of a top emission structure, in which the upper electrode is made transparent and light is emitted from the upper electrode. For example, Patent Document 1 discloses an organic EL element of top emission structure having an upper electrode of 2-layer structure, with the first layer of Mg, Ag or the like working as an injection layer and second layer of ITO (indium tin oxide) or the like working as a transparent electrode.
Patent Document 2 discloses an active matrix type organic light-emitting display apparatus characterized by its pixel structure with a partition provided on the position at which a driving element electrode is connected to a lower electrode for an organic light-emitting element working as a pixel. The document also discloses that the above structure is applicable to a display apparatus which emits light from the upper electrode side.
As an attempt to prevent lowered brightness of emitted light resulting from distance from the point at which power is supplied to the electrode, Patent Document 3 discloses a structure with an auxiliary power source line of low resistance material, provided beside the pixel and connected to the upper transparent electrode.
Patent Document 4 discloses a structure which realizes full-color display using a color filter or color conversion filter, where all of the pixels of top emission structure are devised to emit white or blue color.    Patent Document 1: U.S. Pat. No. 5,703,436    Patent Document 2: JP-A-2001-148291    Patent Document 3: JP-A-2001-230086    Patent Document 4: JP-A-2001-217072